The present invention relates to the transmission of telecommunications data, and more particularly to a novel method of transmitting data on fractional time slots.
Asynchronous Transfer Mode (ATM) is a standard protocol for transmitting asynchronous telecommunications data. This protocol is based on the transmission of data in fixed size data packets known as ATM cells. Each ATM cell exhibits a singular format comprising a 48 octet payload portion and a five octet header portion. ATM is well known.
Unfortunately, ATM does not efficiently transport low bit rate data as the length of a typical low bit rate data packet is significantly less than 48 octets (i.e., the length of an ATM cell payload). Any unused portion of an ATM cell payload is filled with xe2x80x9cpadding bitsxe2x80x9d. When padding bits are inserted rather than data, bandwidth is wasted. The insertion of padding bits may also result in unacceptable transmission delays, which may be detrimental, especially when the data being transported is highly sensitive to delays, such as voice-type data.
An ATM adaptation layer, known as AAL2, has been developed for the purposes of carrying compressed voice data on ATM and for improving the efficiency of ATM when employed to transport low bit rate data according to Recommendation I.363.2 (hereinafter xe2x80x9cI.363.2xe2x80x9d) which has been approved by the International Telecommunications Union (ITU). Referring to FIG. 1, AAL2 operates by storing low bit rate data in small, variable length data packets called minicells, for example, minicells 191-197, which are sometimes referred to as microcells or short packets. An improvement in bandwidth utilization is achieved by inserting several minicells into the payload of a single ATM cell, such as ATM cell 101. To further improve bandwidth utilization, a minicell, for example minicell 193, may be segmented so that it overlaps two ATM cells 101 and 102 as illustrated.
Under this standard, ATM is needed as an underlying bearer and is defined to be carried on the E1 line in 30 time slots (1-15 and 17-31) and on the T1 line on all 24 time slots. These standards are according to ITU-T Recommendation G.804.
In some cases, using the whole T1 or E1 can be prohibitively expensive. There are an insufficient number of AAL2 connections to utilize the entire bandwidth. In these cases, fractional T1 or E1 may be utilized. Fractional implies that a number of time slots are concatenated to form a channel, e.g., six concatenated time slots comprising 384 Kbps of bandwidth is quite common. The cost per bit of transported information increases as the amount of bandwidth that is utilized decreases. For instance, using a fractional E1 or T1 with ATM and AAL2 results in a bandwidth penalty of about 10%.
FIG. 1 illustrates a conventional AAL2 multiplexing technique with added resilience against loss of delineation in the form of a start octet.
The basic delineation between cells is provided by fixed size ATM cells 101, 102 and 103. The fact that the ATM cells come xe2x80x9cback to backxe2x80x9d every 53 octets makes it easy to use a receiver state machine that takes this into account. An ATM header 111 of five octets contains a header error control (HEC) field 121 that makes it possible for the receiver to check the integrity of the ATM cell header. Under normal practice, if six ATM cell headers in a sequence are received without any errors, the receiver is considered to be synchronized. Furthermore, due to the 53 octet length, the state machine does not have to leave the sync state at a first error in the ATM cell header. If the error is repeated a predetermined number of times such as, for example, six times, however, it is considered to no longer be in the synchronized state. The same technique is more difficult to apply to the AAL2 demultiplexing since the minicells 191 to 197 can have variable sizes. A length indicator (LI) field 151 provided in the header of each minicell is used to find the start of the next minicell. The entire minicell is protected by a HEC that is similar to the HEC for the ATM cell. This ensures that the integrity of LI can be checked.
In addition, an offset field 123 of six binary coded bits is inserted as a first octet in the payload of every ATM cell. The offset field contains a pointer that makes it possible to find the first minicell, at every new ATM cell, regardless of the LI value 151. The pointer is encapsulated in a start octet 119. The start octet 119 also includes a sequence number bit 125, working as a modulo-2 counter, making it possible to detect if ATM cells have been lost or if there is only a single cell. The start octet 119 is protected by a parity bit 127. If no remaining minicells exist to fill an ATM cell, the remainder of the cell is padded by inserting a zero in every octet to the end of the ATM cell.
FIG. 2 illustrates a minicell according to I.363.2. This packet is made up of a connection identifier (CID) field 205, a length indicator (LI) field 211, a user to user indication (UUI) field 215, a header error control (HEC) field 221 and a payload field 251.
The CID field 205 is eight bits in length allowing up to 255 connections ranging from CID1, to CID255. CID0 is reserved for padding, i.e., if the next octet after the last octet in a previous minicell is zero, then the remainder of the ATM cell is empty. In other words, if the octet where a new minicell is supposed to start is zero, then the remaining octets in the ATM cell are filled with zeroes which is considered to be padding. The receiver, when it detects a zero octet where a new minicell is supposed to start, disregards the remainder of the ATM cell. The LI field 211 is six bits in length and indicates the number of octets in the payload. It ranges from LI0 to LI44 which corresponds to payloads of one to 45 octets. The UUI field 215 is also five bits in length and is transparently conveyed from one end user to the other end user. Transparency, in this context, means that the user may or may not be aware of this activity, in this case, the UUI field being conveyed. It may be considered as a field in which the user may place any type of information as long as that information is not placed in the range of UUI26 to UUI31 which are reserved for segmentation and OAM usage. The HEC field 221, also five bits in length, may be used to verify the integrity of the minicell header.
A copending application, Ser. No. 08/982,425 for xe2x80x9cSimultaneous Voice And Dataxe2x80x9d of Petersen et al., discloses delineation without the support of the underlying ATM connection. The subject matter of this application is hereby incorporated by reference. The described method of delineation, however, is only sufficient for a limited number of active connections and a low bandwidth line, of typically less than 64 Kbps. For larger bandwidths associated with fractional T1/E1, this method of delineation is not resilient as a considerable amount of time is needed to achieve resynchronization. In some instances, this time period may even be indefinite. The use of the disclosed method of delineation is appropriate in a private access line with few users. For public channels with many users expecting a certain quality, however, the time needed to achieve resynchronization is unacceptable. What is needed is another method for achieving re-synchronization in a shorter period of time. This may be accomplished by adapting the AAL2 packets to work directly on fractional time slots.
FIGS. 3a and 3b illustrate a conventional synchronous frame with T1 time slot structure of 1544 Kbps and E1 time slot structure of 2048 Kbps respectively. The T1 channel structure is divided into 24 consecutive time slots 305 preceded by a F-bit 307. The time frame is repeated every 125 microseconds. The F-bit 307 is used to indicate, among other things, the start of the frame and multi-frame. The multi-frame structure 309 is repeated after 24 frames.
The E1 channel structure has a similar channel structure based on 32 consecutive time slots 315. TS0 is used to indicate, among others, the start of the frame and multi-frame. The multi-frame structure 319 in this case is repeated after 16 frames. In the E1 channel structure, time slot 16 is predefined for signaling purposes.
In the frame and multi-frame, codes are carried for identifying the start of the frame and the multi-frame and therefore, the position of the individual time slots can also be identified. As a result, the receiver can keep track of the all free time slots in the frame and the multi-frame. A given time slot in each frame belongs to the same connection, thus forming a digital channel of 64 Kbps.
The specifics of the frame, multi-frame and channel structure and allocation can be found in ITU-T Recommendation G.704. In addition, synchronization codes are provided so that the start of the frame and multi-frame can also be determined.
FIGS. 3a and 3b also illustrate the concatenation of six 64 Kbps time slots in a frame to form one channel of 384 Kbps. Four such channels in the T1 and five channels in the E1 are depicted.
The number of time slots that can be concatenated to form one channel is limited to the maximum number of time slots in the frame (with respect to the predefined TS 0 and TS15 in E1). For an E1 structure, there are 32 time slots numbered TS0 to TS31. Of these, 30 time slots, TS1-TS15 and TS17-TS31 may be used for carrying data or control information. As the multi-frame is made up of 16 frames, there are a total of 512 time slots (16 frames each having 32 time slots) and 480 (16 frames with 30 available time slots) of these time slots are available for carrying data or control information.
For the T1 structure, there are 24 time slots all of which are available for carrying data or control information resulting in a total of 576 time slots (24 frames each having 24 time slots). In the T1, an additional bit is added to provide frame and multi-frame synchronization.
The synchronization support in both E1 and T1 enables a receiver to detect the start of the frame and the multi-frame. Consequently, the receiver can keep track of all free time slots in the frame and the multi-frame.
The frames are repeated at a rate of 8 kHz or every 125 micro seconds. The E1 multi-frame, therefore, is repeated after 2 milliseconds (16 frames each repeating every 125 microseconds) and the T1 is repeated after 3 milliseconds (24 frames each repeating every 125 microseconds).
As long as the number of time slots fit within the frame, channels of any bandwidth can be structured in this manner. The channels can even be of varying bandwidth provided that the maximum allowed number of time slots is not exceeded. The concatenated time slots can come in any predetermined order even if consecutive allocation is preferred.
In the illustrations of FIGS. 3a and 3b, any of the 384 Kbps channels can carry minicells. Since 384 Kbps is a very limited bandwidth, a gain could be made by eliminating the ATM cells that are needed to carry the minicells according to I.363.2. The copending application referred to earlier discloses the delineation method based on AAL2 without the ATM layer. If the delineation is lost, the recovery time can be extremely long and may even be indefinite, without the ATM layer.
What is needed, therefore, is AAL2 working directly on fractional T1/E1 time slots without the need for ATM as an underlying bearer in order to save the 10% bandwidth penalty that is associated with ATM.
According to an exemplary embodiment of the present invention, a method of mapping a plurality of minicells into a channel structure is disclosed. The method comprises the steps of: inserting a start octet in a first of a plurality of time slots corresponding to the channel wherein the plurality of time slots comprise a multi-frame; and counting a number of multi-frames using a sequence number in the start octet wherein a plurality of multi-frames comprise the channel structure.
According to another exemplary embodiment of the present invention, a method of mapping a plurality of minicells into a channel structure having a plurality of multi frames with each of the multi frames having a plurality of time slots is disclosed. This method comprises the steps of: inserting a start octet in a first time slot of the plurality of time slots within each of the multi-frames; counting a number of multi-frames using a sequence number in the start octet; and inserting a connection identifier having a zero value at the end of a transmission of each of the packets.
Exemplary embodiments of the present invention also disclose a method for allocating fractional time slots for carrying data in AAL2 format. The method comprises the steps of: generating, by a channel structure framer, a framing sequence comprising one of a multi-frame signal and a time slot signal; sending the sequence to a time slot counter; generating, by the time slot counter, an address to a location in a time slot table which location includes a channel number and a qualifier; and activating one of a padding generator, a start octet generator and a minicell using the channel number and the qualifier.
Other exemplary embodiments of the present invention disclose a method for providing link synchronization of fractional time slots which have been allocated for carrying AAL2 format data without using ATM as underlying bearer. The method comprises the steps of: generating, by a channel structure framer, one of a multi-frame signal and a time slot signal; transmitting the generated signal to a time slot counter; generating, by the time slot counter, an address to a location in a time slot table which location includes a channel number and a qualifier; and determining, using contents of the address location in the time slot table, a channel to which the time slot belongs.